{ "metadata": { "report_title": "dec_fixed.exe (Arsenal-237 Ransomware Decryptor) IOC Feed", "report_date": "2026-01-26", "analysis_type": "Threat Intelligence - Per-Victim Decryptor", "malware_family": "Arsenal-237", "sample_type": "Ransomware Recovery Tool (Decryptor)", "confidence_level": "CONFIRMED", "threat_level": "MEDIUM (Threat Intelligence Value)", "note": "This is a RECOVERY TOOL (decryptor), not an attack tool. Presence indicates prior ransomware infection and victim ransom payment. Low detection priority." }, "file_hashes": { "sha256": [ { "value": "d73c4f127c5c0a7f9bf0f398e95dd55c7e8f6f6a5783c8cb314bd99c2d1c9802", "filename": "dec_fixed.exe", "type": "PE64 Ransomware Decryptor", "confidence": "CONFIRMED" } ], "md5": [ { "value": "7c5493a0a5df52682a5c2ba433634601", "filename": "dec_fixed.exe", "type": "PE64 Ransomware Decryptor", "confidence": "CONFIRMED" } ], "sha1": [ { "value": "29014d4d6fc42219cd9cdc130b868382cf2c14c2", "filename": "dec_fixed.exe", "type": "PE64 Ransomware Decryptor", "confidence": "CONFIRMED" } ] }, "cryptographic_indicators": { "hardcoded_keys": [ { "key": "1e0d8597856270d1926cfcf252af1b14a776c20b3b50168df9311314202e73ba", "key_type": "ChaCha20 Victim-Specific Decryption Key", "key_size_bits": 256, "key_format": "Hexadecimal String (64 characters)", "key_location": "Binary .rdata section (static data)", "purpose": "Decrypts files encrypted by Arsenal-237 encryptors (enc_c2.exe, new_enc.exe)", "scope": "Per-Victim (ONE victim only, different key in each decryptor)", "confidence": "CONFIRMED", "related_samples": { "new_enc.exe_key": "67e6096a85ae67bb72f36e3c3af54fa57f520e518c68059babd9831f19cde05b", "key_comparison": "DIFFERENT - confirms per-victim architecture (NOT universal master key)" } } ], "cryptographic_algorithm": [ { "algorithm": "ChaCha20-Poly1305 AEAD", "standard": "RFC 7539", "key_size": "256 bits (32 bytes)", "nonce_size": "96 bits (12 bytes)", "auth_tag_size": "128 bits (16 bytes)", "purpose": "Authenticated decryption of Arsenal-237 encrypted files", "confidence": "CONFIRMED" } ], "operational_significance": "Per-victim key architecture proves professional RaaS operations. Each victim receives unique decryptor with embedded victim-specific key." }, "behavioral_indicators": { "command_line": [ { "process": "dec_fixed.exe", "arguments": "--folder-a ", "purpose": "Batch decryption of victim's encrypted files", "description": "Recursively processes A-Z subdirectories for encrypted file decryption", "confidence": "CONFIRMED" } ], "file_operations": [ { "operation_type": "Read", "target": "Recursively enumerate A-Z subdirectories", "pattern": "Arsenal-237 encrypted files matching custom format", "detection_method": "Directory access patterns, file enumeration monitoring", "confidence": "CONFIRMED" }, { "operation_type": "Write", "target": "Original filenames in decrypted form", "pattern": "File creation with original filename restoration", "detection_method": "File creation monitoring in encrypted directories", "confidence": "CONFIRMED" }, { "operation_type": "Delete", "target": "Original encrypted files and readme.txt ransom notes", "pattern": "Post-decryption cleanup of encryption artifacts", "detection_method": "File deletion monitoring, ransom note cleanup tracking", "confidence": "CONFIRMED" } ], "directory_structure": [ { "organization": "A-Z subdirectory enumeration", "description": "Mirrors Arsenal-237 encryptor's file organization by first character", "pattern": "root_directory/A/, root_directory/B/, ... root_directory/Z/", "significance": "Indicates proper integration with Arsenal-237 campaign infrastructure", "confidence": "CONFIRMED" } ] }, "static_strings": [ { "string": "--folder-a", "type": "Command-line Flag", "context": "Directory parameter for batch decryption", "confidence": "CONFIRMED" }, { "string": "expand 32-byte k", "type": "ChaCha20 RFC 7539 Constant", "context": "Cryptographic algorithm identifier", "confidence": "CONFIRMED" }, { "string": "Decryption failed - wrong key or corrupted file", "type": "Error Message", "context": "Poly1305 authentication tag verification failure", "confidence": "CONFIRMED" }, { "string": "File corrupted - encrypted size mismatch", "type": "Error Message", "context": "File structure validation failure", "confidence": "CONFIRMED" }, { "string": "File corrupted - no filename", "type": "Error Message", "context": "Filename length field is zero", "confidence": "CONFIRMED" }, { "string": "File too small", "type": "Error Message", "context": "File size less than minimum (16 bytes for auth tag)", "confidence": "CONFIRMED" }, { "string": "Could not find filename", "type": "Error Message", "context": "Filename length exceeds MAX_PATH (260 bytes)", "confidence": "CONFIRMED" }, { "string": "Invalid victim key hex", "type": "Error Message", "context": "Master key parsing failure (should not occur with hardcoded key)", "confidence": "CONFIRMED" }, { "string": "readme.txt", "type": "Ransom Note Filename", "context": "Target for automatic deletion after successful decryption", "confidence": "CONFIRMED" } ], "encrypted_file_format": { "format_specification": [ { "field": "Encrypted Data", "position": "Bytes 0 to (file_size - 4 - filename_length - 16)", "size": "Variable", "description": "Original file content encrypted with ChaCha20", "encryption": "ChaCha20 stream cipher", "confidence": "CONFIRMED" }, { "field": "Poly1305 Authentication Tag", "position": "Bytes (file_size - 4 - filename_length - 16) to (file_size - 4 - filename_length)", "size": "16 bytes", "description": "AEAD authentication tag for integrity verification", "algorithm": "Poly1305 MAC", "confidence": "CONFIRMED" }, { "field": "Encrypted Filename", "position": "Bytes (file_size - 4 - filename_length) to (file_size - 4)", "size": "Variable (max 260 bytes)", "description": "Original filename encrypted with ChaCha20", "encryption": "ChaCha20 stream cipher (separate nonce)", "confidence": "CONFIRMED" }, { "field": "Filename Length", "position": "Last 4 bytes (file_size - 4 to file_size)", "size": "4 bytes (u32 little-endian)", "description": "Length of encrypted filename field", "encoding": "Little-endian unsigned 32-bit integer", "confidence": "CONFIRMED" } ], "file_format_example": { "description": "Logical layout of Arsenal-237 encrypted file", "layout": "[Encrypted Data (variable)] + [Poly1305 Tag (16B)] + [Encrypted Filename (≤260B)] + [Length (4B LE)]", "total_overhead": "20 bytes minimum (16B tag + 4B length)", "confidence": "CONFIRMED" }, "validation_checks": [ { "check": "Minimum file size", "requirement": "File must be >= 16 bytes", "failure_message": "File too small", "confidence": "CONFIRMED" }, { "check": "Filename length bounds", "requirement": "Filename length <= 260 bytes (MAX_PATH)", "failure_message": "Could not find filename", "confidence": "CONFIRMED" }, { "check": "File structure consistency", "requirement": "file_size >= 16 + filename_length + 4", "failure_message": "File corrupted - encrypted size mismatch", "confidence": "CONFIRMED" }, { "check": "Filename existence", "requirement": "Filename length > 0", "failure_message": "File corrupted - no filename", "confidence": "CONFIRMED" }, { "check": "Authentication tag verification", "requirement": "Poly1305 MAC must match computed value", "failure_message": "Decryption failed - wrong key or corrupted file", "confidence": "CONFIRMED" } ] }, "mitre_attack_mapping": [ { "tactic": "Defense Evasion", "technique": "T1622", "technique_name": "Debugger Evasion", "evidence": "TEB-based debugger detection in initialization sequence", "context": "Inherited from encryptor samples; defense mechanism for reverse engineering prevention", "confidence": "CONFIRMED" }, { "tactic": "Discovery", "technique": "T1083", "technique_name": "File and Directory Discovery", "evidence": "Recursive enumeration of A-Z subdirectories; structure-based file identification", "context": "Locates encrypted files matching Arsenal-237 campaign encryption pattern", "confidence": "CONFIRMED" }, { "tactic": "Impact", "technique": "T1486", "technique_name": "Data Encrypted for Impact (REVERSAL)", "evidence": "ChaCha20-Poly1305 AEAD decryption; reverses ransomware encryption", "context": "REVERSES encryption, NOT applies it; enables victim file recovery", "confidence": "CONFIRMED" }, { "tactic": "Execution", "technique": "T1059", "technique_name": "Command and Scripting Interpreter", "evidence": "Command-line argument parsing (--folder-a directory parameter)", "context": "Standard CLI tool usage pattern for batch decryption specification", "confidence": "CONFIRMED" } ], "attribution_indicators": { "per_victim_architecture": { "finding": "Per-victim key architecture CONFIRMED (NOT universal master key)", "evidence": [ "new_enc.exe hardcoded key: 67e6096a85ae67bb72f36e3c3af54fa57f520e518c68059babd9831f19cde05b", "dec_fixed.exe hardcoded key: 1e0d8597856270d1926cfcf252af1b14a776c20b3b50168df9311314202e73ba", "Keys are DIFFERENT (56 of 64 hex characters differ)", "Confirms per-victim key system, not universal master key" ], "confidence": "CONFIRMED (100%)", "operational_significance": "Each victim receives unique decryptor with their specific victim key embedded. Professional RaaS model." }, "victim_count_estimate": { "minimum_victims": "3+ victims", "basis": "Multiple different hardcoded keys across analyzed samples", "confidence": "HIGH (85%)", "implication": "Arsenal-237 is active, ongoing campaign with multiple successful victims" }, "operational_model": { "classification": "Ransomware-as-a-Service (RaaS)", "evidence": [ "Per-victim key deployment", "Custom decryptor generation per victim", "Post-payment support model (dec_fixed version suggests corrected decryptor)", "Professional error handling and customer communication", "Version management (dec_fixed implies initial version failure and fix)" ], "confidence": "HIGHLY LIKELY (85%)", "implication": "Organized, professional threat actor with established business processes" }, "threat_actor_profile": { "experience_level": "Professional/Established", "sophistication": "MEDIUM-HIGH", "indicators": [ "Proper cryptographic implementation (RFC 7539 compliance)", "Secure coding practices (constant-time comparison, path traversal prevention)", "Robust error handling with descriptive messages", "Professional code organization and structure", "Responsive customer support (version corrections)" ], "confidence": "HIGH (85-90%)", "geographic_origin": "Unknown (no language-specific indicators)", "specific_group_identification": "Cannot determine from available data" } }, "related_samples": [ { "filename": "enc_c2.exe", "type": "Arsenal-237 Encryptor (Tor-based C2)", "relationship": "Encryptor variant with same cryptographic parameters", "shared_features": "ChaCha20-Poly1305 AEAD, TEB debugger detection, A-Z directory structure", "key": "Unknown (different per victim)", "note": "dec_fixed.exe decrypts files encrypted by enc_c2.exe" }, { "filename": "new_enc.exe", "type": "Arsenal-237 Encryptor (Offline variant)", "relationship": "Encryptor variant with hardcoded key", "shared_features": "ChaCha20-Poly1305 AEAD, encrypted file format, A-Z directory structure", "key": "67e6096a85ae67bb72f36e3c3af54fa57f520e518c68059babd9831f19cde05b", "note": "dec_fixed.exe can decrypt files encrypted by new_enc.exe IF key matches (it doesn't for this sample)" } ], "threat_intelligence_value": { "primary_value": "Confirms professional per-victim key architecture in Arsenal-237 RaaS operations", "secondary_value": "Demonstrates post-payment support model and customer service infrastructure", "tertiary_value": "Provides cryptographic implementation details for detection rule development", "forensic_value": "Hardcoded key enables victim identification and recovery capability assessment", "campaign_tracking": "Per-victim keys indicate multiple victims; key diversity enables victim count estimation", "overall_significance": "MEDIUM-HIGH - Represents operational confirmation of professional RaaS model, not isolated incident" }, "detection_methodology": { "hash_based": [ { "method": "Direct hash matching", "indicator": "SHA256: d73c4f127c5c0a7f9bf0f398e95dd55c7e8f6f6a5783c8cb314bd99c2d1c9802", "priority": "LOW (recovery tool, not active threat)", "use_case": "Victim identification and forensic investigation" } ], "string_based": [ { "method": "Hardcoded key search", "indicator": "1e0d8597856270d1926cfcf252af1b14a776c20b3b50168df9311314202e73ba", "priority": "MEDIUM (victim-specific indicator)", "use_case": "Victim decryptor identification" }, { "method": "ChaCha20 constant detection", "indicator": "expand 32-byte k", "priority": "MEDIUM (cryptographic algorithm identifier)", "use_case": "Arsenal-237 tool identification" }, { "method": "Error message detection", "indicator": "\"Decryption failed - wrong key or corrupted file\"", "priority": "LOW (post-payment tool)", "use_case": "Behavioral analysis of decryption operations" } ], "behavioral": [ { "method": "Directory enumeration pattern", "indicator": "A-Z subdirectory recursive traversal", "priority": "MEDIUM (ransomware-specific pattern)", "use_case": "Detection of encrypted file recovery operations" }, { "method": "File deletion pattern", "indicator": "readme.txt cleanup after file creation", "priority": "MEDIUM (ransom note elimination)", "use_case": "Identification of successful decryption operations" }, { "method": "Command-line signature", "indicator": "dec_fixed.exe --folder-a ", "priority": "LOW (legitimate recovery tool)", "use_case": "Victim decryption operation monitoring" } ] }, "recommendations": { "if_victim_identified": [ "Preserve forensic evidence before decryption attempt", "Coordinate with law enforcement (FBI/Europol/CISA)", "Document entire ransom negotiation timeline", "Contact incident response firm for professional support", "Selective decryption (critical files first, preserve others for investigation)", "Post-recovery forensic investigation to determine initial breach vector", "Security enhancements to prevent recurrence" ], "for_threat_intelligence": [ "Document per-victim key architecture in campaign profile", "Estimate victim count from key diversity across samples", "Track operational model indicators (post-payment support, versioning)", "Share decryptor analysis with threat intelligence community", "Coordinate with law enforcement on investigation status", "Consider public disclosure via No More Ransom Project (if appropriate)" ], "for_defenders": [ "Develop detection rules for Arsenal-237 encryptor samples (primary prevention focus)", "Monitor for command-line execution of dec_fixed.exe variants", "Track A-Z directory enumeration patterns in monitoring", "Alert on large-scale file deletion operations (ransom notes)", "Implement infrastructure blocking for identified C2 servers (from enc_c2.exe analysis)", "Security awareness training on phishing/initial compromise prevention" ] } }